diff --git a/packages/core/src/tools/web-fetch.ts b/packages/core/src/tools/web-fetch.ts
index 3d631af9b1..3a865783d5 100644
--- a/packages/core/src/tools/web-fetch.ts
+++ b/packages/core/src/tools/web-fetch.ts
@@ -410,7 +410,7 @@ export class WebFetchTool extends BaseDeclarativeTool<
         properties: {
           prompt: {
             description:
-              'A comprehensive prompt that includes the URL(s) (up to 20) to fetch and specific instructions on how to process their content (e.g., "Summarize https://example.com/article and extract key points from https://another.com/data"). Must contain as least one URL starting with http:// or https://.',
+              'A comprehensive prompt that includes the URL(s) (up to 20) to fetch and specific instructions on how to process their content (e.g., "Summarize https://example.com/article and extract key points from https://another.com/data"). All URLs to be fetched must be valid and complete, starting with "http://" or "https://", and be fully-formed with a valid hostname (e.g., a domain name like "example.com" or an IP address). For example, "https://example.com" is valid, but "example.com" is not.',
             type: 'string',
           },
         },